Wet scrubbing method and apparatus for removing sulfur oxides from combustion effluents

Abstract

Sulfur oxides (SO x ) are scrubbed from combustion effluents with aqueous limestone slurries with greater efficiency and with economies in capital and operating costs. In the preferred embodiment of a single-loop, open-tower countercurrent limestone wet scrubber, effluent flow rates are greatly increased while L/G values and reaction tank residence times are decreased. Improved entrainment eliminator design, novel nozzle placement and spacing, and the use of a hydrocyclone to separate and recycle smaller particles of limestone from the byproduct gypsum, facilitate these advantages. The limestone is reduced to very fine particles, e.g. about 8μ or less with more than 99% of the particle by weight less than 44μ, and introduced into a scrubbing slurry which is contacted with SO x -laden effluent.

We claim: 
     
       1. A single-loop, open-tower, countercurrent limestone wet scrubbing process for reducing the concentration of SO x  in flue gases, comprising: (a) directing a flow of flue gas containing SO x  upwardly through an open vertical scrubbing tower at a bulk flow velocity of from 4.5 meters per second to up to about 6 meters per second;   (b) introducing into a vertical scrubbing section within said tower, a spray of droplets of an aqueous slurry of finely-divided calcium carbonate, calcium sulfate, dissolved calcium and inert solids to contact the flue gas while descending through the tower counter currently to the flow of flue gas to absorb SO x  from the flue gas, the pH of the slurry as introduced into the scrubbing tower being within the range of from about 5.0 to about 6.3, the slurry is introduced by spray nozzles, arranged in two to three levels with a spacing between levels of less than about 2 meters, and with the direction of flow from adjacent nozzles alternating between upward and downward;   (c) collecting the slurry containing dissolved SO x  in a reaction tank after contact with the flue gas;   (d) withdrawing slurry from the reaction tank after an average residence time of less than about 8 hours but sufficient to permit reaction of dissolved SO x  with dissolved calcium and growth of crystals of calcium sulfate to a weight median diameter of at least 2 times as large as the calcium carbonate added as feed;   (e) subjecting slurry withdrawn from the reaction tank to a treatment effective to provide a recycle stream rich in fine particles of calcium carbonate having a weight median diameter of about 8μ or less and a molar ratio of calcium-containing to sulfur-containing compounds of at least 1.3, and another stream rich in relatively larger particles of calcium sulfate particles having a weight median diameter of at least 25μ;   (f) discharging the stream rich in calcium sulfate particles to thereby remove SO x  previously dissolved in the slurry, and returning to the process a major portion of the recycle stream rich in calcium carbonate and poor in calcium sulfate; and   (g) introducing fresh calcium carbonate as feed into the system in amounts sufficient to maintain the pH of the slurry in the reaction tank within the range of from 5.0 to 6.3 and to replace the calcium withdrawn and not recycled as well as that dissolved and reacted with the SO x  absorbed in the liquid phase in the scrubbing section, the finely-divided calcium carbonate introduced as feed having a weight median particle size of less than about 10μ as introduced.   
     
     
       2. A process according to claim 1 wherein finely-divided calcium carbonate introduced as feed has a weight median particle size of less than about 8μ as introduced. 
     
     
       3. A process according to claim 1 wherein the tower comprises a single pass entrainment separator effective to reduce the quantity of droplets and to turn the direction of flow of the flue gases from the vertical to an orientation effective for efficient utilization of a vertically-oriented mist separator, wherein the entrainment separator includes individual single-pass separator blades, said individual blades mounted to form assemblies, said individual blades are oriented at an angle from the vertical. 
     
     
       4. A process according to claim 3 wherein the tower further comprises a vertically-oriented mist eliminator, and said entrainment eliminator being effective to turn the direction of flow of the flue gases by at least 30° from the vertical axis of the tower. 
     
     
       5. A process according to claim 1 wherein the slurry withdrawn from the reaction tank is passed to a hydrocyclone for separation. 
     
     
       6. A process according to claim 1 wherein the portion of the slurry in the recycle stream which is fed back the reaction tank has a solids concentration of less than 10%. 
     
     
       7. A process according to claim 6 wherein the molar ratio of calcium-containing to sulfur-containing compounds in the recycle stream is greater than about 1.4. 
     
     
       8. A process according to claim 6 wherein the recycle stream comprises less than 5% suspended solids. 
     
     
       9. A process according to claim 1 wherein the median size of the calcium carbonate particles in the reaction tank is within the range of from about 2 to about 6μ, and the weight median particle size of the finely-divided calcium carbonate as introduced is less than about 8μ, with 99% by weight of the particles being less than 44μ. 
     
     
       10. A process according to claim 1 wherein the pH of the slurry in the reaction tank is maintained within the range of from about 5.8 to about 6.3. 
     
     
       11. A process according to claim 1 wherein the molar ratio of calcium-containing to sulfur-containing compounds in the recycle stream is greater than about 1.4, and the recycle stream comprises less than 5% suspended solids. 
     
     
       12. A process according to claim 1 wherein air is supplied to the scrubbing tower to supply oxygen for the oxidation of sulfite and bisulfite ions. 
     
     
       13. A process according to claim 1 wherein the weight median particle size of the calcium carbonate in the droplets of slurry introduced into the scrubbing section is about 6μ or less. 
     
     
       14. A process according to claim 1 wherein the average residence time in the reaction tank is less than six hours.